Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The lasing material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (Kozlov et al., U.S. Pat. No. 6,160,828), waveguide, ring microlasers, and distributed feedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog. Phys. 63, 729 (2000) and M. Diaz-Garcia et al., U.S. Pat. No. 5,881,083). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically-pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V-s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (V. G. Kozlov et al., J. Appl. Phys. 84, 4096 (1998)). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude more charge carriers than single excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (N. Tessler et al., Appl. Phys. Lett. 74, 2764 (1999)). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (M. Berggren et al., Nature 389, 466 (1997)), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (N. Tessler, Adv. Mater. 10, 64 (1998)).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. One of the advantages of organic-based lasers is that since the material is typically amorphous, the devices can be formed inexpensively and they can be grown on any type of substrate. The single-crystal organic-laser approach obviates both of these advantages.
A few others have suggested pumping the organic laser cavity with light-emitting diodes (LED's), either inorganic (M. D. McGehee et al., Appl. Phys. Lett. 72, 1536 (1998)) or organic (Berggren et al., U.S. Pat. No. 5,881,089). McGehee et al. (M. D. McGehee et al., Appl. Phys. Lett. 72, 1536 (1998)) state that they needed to lower their thresholds by at least an order of magnitude to attempt laser pumping using an InGaN LED. Berggren et al. propose making an all organic unitary laser where one section of the device (the organic LED part) provides the incoherent radiation, while the adjacent section (the laser cavity) provides optical down conversion, gain and optical feedback. Berggren et al. state that the lasing cavity should be either a waveguide with facets, a distributed-feedback waveguide cavity, a distributed-Bragg-reflector waveguide cavity, or a photonic-lattice cavity. Berggren et al. only showed data for the organic light-emitting diode (OLED) section of the device (its current-voltage and voltage-luminance characteristics). With respect to the device's lasing characteristics, their only comment was that it produced coherent radiation at ˜620 nm. Since Berggren et al. never gave any additional details with respect to the device's lasing operation, it is difficult to determine if the device lased as a result of excitation from the OLED section of the device. Consequently, to the best of our knowledge, there have not been any documented cases of laser cavities excited by incoherent light sources.